U.S. Pat. No. 6,185,158 in particular discloses an electronic watch which is equipped with an analogue display of multiple time variables, in particular the hour, the minute and the second by means of three coaxial hands which are located in the centre of the face of the watch. Furthermore, the analogue display comprises a stopwatch hand, in particular a minute hand indicating the measured interval of time, which is associated with a circular graduation over 360° as well as a ring of dates, the date conventionally appearing in a window of the face. This document proposes actuating the mechanism for driving the stopwatch hand (hereinafter referred to as the first mechanism), and the mechanism for driving the ring of dates (hereinafter referred to as the second mechanism) by one and the same electromagnetic motor. In the context of the described embodiment, it is provided that the first mechanism is not associated with the display of the current hour, minute or second, whereas the second mechanism must be of the type which is driven periodically or intermittently, as is the case for the display of the date.
The first mechanism comprises an intermediate wheel which is driven directly by the rotor of the motor and a chronograph wheel which meshes with the intermediate wheel. The second mechanism likewise comprises said intermediate wheel and further comprises an auxiliary wheel which meshes with said intermediate wheel. The auxiliary wheel is rigidly connected to a movable element for periodically actuating a wheel for driving the ring of dates, said movable element having a finger for actuating said drive wheel. The movable element for periodic actuation and the drive wheel together form a Geneva drive, which is known for periodically driving a ring/disc of dates. The Geneva drive is characterised by periodic driving of the wheel for driving the ring of dates, the movable element for periodic actuation meshing with said drive wheel only over a limited angular sector which is certainly less than 360°, while said movable element blocks the drive wheel over the complementary angular sector by means of a circular lateral surface. Thus, although the movable element for periodic actuation rotates while it is positioned in the complementary angular sector, the rotational movement of the rotor is not transmitted to the ring of dates.
The Geneva drive has at least two main functions. Firstly, this mechanism is designed to keep the wheel for driving the ring of dates, and thus the ring of dates, stationary over a specific angle distance from the movable element for periodic actuation (for example approximately 300°) as mentioned above. Secondly, this mechanism has a shock-resistance function when the drive wheel is stationary. Said drive wheel and the movable element for periodic actuation are arranged in such a way that the movable element for periodic actuation blocks any rotation of the drive wheel when said element is not setting said wheel into rotation. Since the meshing between the drive wheel and the set of teeth of the ring of dates is provided almost without any clearance (except for that of the conventional machining tolerances), the ring of dates is protected in the event of shock and remains, outside periods of driving which are provided for displaying the date, substantially in a stationary position during a shock.
In addition, U.S. Pat. No. 6,185,158 uses a Geneva drive to allow the motor used for driving the date to carry out an additional function, that is to say driving a stopwatch hand, as mentioned above. The method provided in this document consists in driving the stopwatch hand when the movable element for periodic actuation is in the non-actuation region thereof, i.e. in said complementary angular sector, and, at the end of the measured time interval, carrying out a reset in reverse to move the movable element for periodic actuation back to a predefined initial position. For this purpose, it is provided that the stopwatch hand normally carries out only one turn, and that the driving of said stopwatch hand over a whole turn leaves the movable element for periodic actuation in the non-actuation region thereof.
The concept of a multifunctional electromechanical motor making it possible to actuate the indicators of two independent items of information is advantageous because it makes it possible to reduce the number of motors and thus the bulkiness of the motor means in the clock mechanism. In addition, a clock mechanism of this type is less expensive. However, the device for driving the ring of dates and a stopwatch hand provided in U.S. Pat. No. 6,185,158 is relatively complex to produce. The production of a clock mechanism having such a mechanism is expensive because it is necessary to reduce as far as possible the machining and installation tolerances (production tolerances) of said mechanism and of the ring of dates to ensure the shock-resistance function and at the same time ensure positioning of this ring in the display positions thereof, for example centring of each date in the window of a face which is provided for the clock mechanism. The Geneva drive, in order to correctly ensure the functions thereof, must be machined with precision and installed in the clock mechanism whilst avoiding clearances between rotating parts, in particular so that two teeth of the drive wheel slide over the circular lateral surface of the movable element for periodic actuation in the above-mentioned non-actuation region. In addition, the Geneva drive does not ensure a shock-resistance function in the periods of driving the ring of dates, since said periods can be relatively long in a “trailing” drive device such as provided in U.S. Pat. No. 6,185,158.